Theoretical comparison of SiC PiN and Schottky diodes based on power dissipation considerations

In order to select the optimal device for a particular application, designers must carefully analyze the tradeoffs between competing devices. Recent progress in SiC power rectifiers has resulted in the demonstration of high-voltage PiN and Schottky barrier diodes (SBDs). With both technologies maturing, power electronics engineers will soon face the task of selecting between these two devices. Until recently, the choice was simple, since silicon SBDs are only available for relatively low voltage applications. The choice is not as clear when considering SiC diodes, and guidelines for determining the proper application of each are needed. The purpose of this paper is to provide such guidelines, based on an analysis of the most significant tradeoffs involved.     

Long-wavelength GaInNAs(Sb) lasers on GaAs

The boom in fiber-optic communications has caused a high demand for GaAs-based lasers in the 1.3-1.6-μm range. This has led to the introduction of small amounts of nitrogen into InGaAs to reduce the bandgap sufficiently, resulting in a new material that is lattice matched to GaAs. More recently, the addition of Sb has allowed further reduction of the bandgap, leading to the first demonstration of 1.5-μm GaAs-based lasers by the authors. Additional work has focused on the use of GaAs, GaNAs, and now GaNAsSb barriers as cladding for GaInNAsSb quantum wells. We present the results of photoluminescence, as well as in-plane lasers studies, made with these combinations of materials. With GaNAs or GaNAsSb barriers, the blue shift due to post-growth annealing is suppressed, and longer wavelength laser emission is achieved. Long wavelength luminescence out to 1.6 μm from GaInNAsSb quantum wells, with GaNAsSb barriers, was observed. In-plane lasers from these samples yielded lasers operating out to 1.49 μm, a minimum threshold current density of 500 A/cm² per quantum well, a maximum differential quantum efficiency of 75%, and pulsed power up to 350 mW at room temperature     

Solar-fuel fired cogeneration plants—Sizing and costing model for molten salt storage systems

The large-scale utilization of solar energy will be facilitated by economical and efficient energy storage. The proposed energy storage systems have been critically reviewed, and capital cost estimates compared on a common basis. A model for sizing an energy storage system is proposed and used to determine the size range of practical interest. Based on selection criteria and relevant data two storage systems have been investigated: an all sodium system and a molten salt system. The design equations, cost estimates, and correlations indicate that, for the energy storage systems developed to date, in the capacity range of 700–2100 MWh, a molten salt, two-tank isolated-type system is the most cost effective and technically feasible for a solar, central receiver, hybrid cogeneration plant. At the extremes of the above range the unit capital cost for the molten salt storage system was found to be 22.8–26.7 $/kWh of stored energy, compared to 43.0–45.4 $/kWh for the sodium storage system.     

Convergence

Convergence is the idea of merging the duties and responsibilities of physical security with that of digital security to make them part of a common reporting structure--to employ tools and techniques that are parallel if not collinear.     

Array designs for amplitude and phase control of millimeter-wave beams

New array design concepts are described for the phase and amplitude control of millimeter and submillimeter-wave beams. Phase shifter array designs providing increased phase range and wider bandwidth are described. Techniques involving the integration of gain-producing elements as well as tuning elements on a single array are proposed for application to high-performance beam control and beam shaping. These concepts should facilitate the further development of quasi-optical solid state device-based arrays for application to millimeter-wave electronic systems.     

Incorporating statistical information into expert classification systems to reduce classification costs

Interactive expert systems seek relevant information from a user in order to answer a query or to solve a problem that the user has posed. A fundamental design issue for such a system is therefore itsinformation-seeking strategy, which determines the order in which it asks questions or performs experiments to gain the information that it needs to respond to the user. This paper examines the problem of optimal knowledge acquisition through questioning in contexts where it is expensive or time-consuming to obtain the answers to questions. An abstract model of an expert classification system — considered as a set of logical classification rules supplemented by some statistical knowledge about attribute frequencies — is developed and applied to analyze the complexity and to present constructive algorithms for doing probabilistic question-based classification. New heuristics are presented that generalize previous results for optimal identification keys and questionnaires. For an important class of discrete discriminant analysis problems, these heuristics find optimal or near-optimal questioning strategies in a small fraction of the time required by an exact solution algorithm.     

Analysis of information exchange activities to actualize and validate situation awareness during shift changeovers in nuclear power plants

Shift work situations occur in almost all safety‐critical organizations, and the investigations of some catastrophes like Chernobyl, Exxon Valdez, and the Gol/Legacy mid‐air collision indicated that shift work information exchange played an important role during the evolution of the situation before the accidents. Inadequate communications during shift changeovers challenged operators' work in the moments that preceded these accidents, because they got inadequate information about the current situation. Our research focuses on the information exchange activities (verbal, written, and nonverbal) of nuclear power plant control operators during shift changeovers. Our aim is to investigate how verbal exchanges and other representations enable operator crews to share information regarding the events that occurred in the previous shift to achieve adequate situation awareness. Our findings indicated the importance and richness of the information exchange during the shift changeover process to update and validate individual and collective situation awareness, showing that information adequately shared enables the ad hoc configurations of regulation loops and a safer use of simplified strategies that can be understood and be validated by other operators, reducing the occurrence of cognitive overloads and contributing to the construction of a common cognitive ground that enhances system resilience. © 2010 Wiley Periodicals, Inc.     

Kinetic interpretation ofα- andβ-Si3N4 formation from oxide-free high-purity silicon powder

A kinetic analysis of the isothermal nitridation of high-purity oxide-free silicon powder is described. The kinetic analysis suggests that the and polymorphs of Si₃N₄ are formed by separate and parallel reaction paths. This analysis provides for the decoupling and quantitative kinetic interpretation of- and-Si₃N₄ formation reactions. Consistent with existing microstructural and thermodynamic evidence, the-forming reaction is shown to obey a first-order rate law, whereas a phase-boundary controlled rate law describes the-forming reaction. A kinetic model employing these rate laws is developed and is used to predict the/ phase ratio as a function of isothermal reaction temperature and extent of reaction. The/ phase ratios so obtained are shown to be in good agreement with experimental observations made under a variety of reaction conditions.     

Direct synthesis and characterization of the Fe3+ analogue of Naβ″-alumina

A new isomorphic analogue of-alumina, a potassium-free Na-ferrite (Na₂O · 5 (Al_0.10Fe_1.90)O₃, referred to as Na-ferrite), has been synthesized and is characterized using X-ray powder and single crystal diffraction, wet chemical analysis, and transmission electron microscopy (TEM). In addition, Na-aluminate-gallate-ferrate phases were also successfully synthesized, but were not characterized in detail. Na-ferrite grew in a slowly cooling melt or partial melt from 1250° C in air using Na₂CO₃ and Fe₃O₄ (magnetite) as starting materials. The compound was also synthesized using-Fe₂O₃ instead of Fe₃O₄ under the same conditions, but its abundance was much less and-Fe₂O₃ and NaFeO₂ were the dominant phases. The new material (cell dimensionsa = 0.5955,c = 3.5623 nm) is reversibly hygroscopic, andc increases 2% after hydration. TEM images dearly show the 1.19 nm lattice spacing corresponding to the ionic conduction planes, and these planes are observed to be both straight and curled. The origin of the curled lattice planes, the existence of which is also evident in the single crystal precession photographs, is not clear, although we believe it may involve a structural misfit between the ferric oxide spineI block and soda layer.